A potential application of the invention is the testing of certificates or sheet, particularly bank notes, checks, or other sheets of paper for the presence of magnetic features, particularly to check whether the certificates or sheets have special previously applied magnetic features.
The quantity of the magnetic material used on a document of value is normally small. The magnetic stray field of the security thread is therefore only a few A/m at a distance of a few mm from the security thread. Very sensitive sensors are needed to verify such types of low magnetic stray fields. The measuring devices used in practice therefore require a distance that is no more than 1 mm between the document of value and the sensor. In addition, only a few technologies are suitable for identifying such types of weak, locally limited stray fields. The use of differential planar coils (DE 37 38 455 A1), magnetoresistive sensor elements (GB 1 362 105), and technologies, inter alia, which are used for read heads or field plates, are known DE 39 21 420 A1 for example. The sensors used in the measuring devices used in the industry are often designed as multi-channel lines (DE 38 51 078 T2, DE 39 16 978 A1 and DE 690 06 529 T2) or as magnetoresistive sensors (DE 692 23 721 T2).
Documents of value can have soft magnetic security features and/or hard magnetic security features. Hard magnetic security features may be produced from materials with high remanence and high coercive field strength. Soft magnetic security features may be produced from materials with high remanence but low coercive field strength. Permanent magnets in particular are produced from hard magnetic materials. Soft magnetic materials are ferromagnetic materials that are easy to magnetize in a magnetic field. The associated magnetic polarization (magnetization) may be generated, for example by means of an electric current in a current-carrying coil or through the presence of a permanent magnet. Polarization generated in this manner leads to a much higher magnetic flux density in the soft magnetic material than is generated in the air by the externally acting magnetic field. Soft magnetic materials generally have a coercive field strength of less than 1000 A/m. Soft magnetic materials may well have saturation magnetization values that are comparable to hard magnetic materials, however, which means that they cannot be differentiated from hard magnetic materials in the saturated condition.
Therefore, in order to verify soft magnetic security features in a document of value, there must be an external magnetic field present that will saturate the soft magnetic substances to the extent possible. Large magnets are installed in known measuring devices in the industry that generate a sufficiently strong field for magnetizing the soft and hard magnetic security features, but that also make the setting up of such types of measuring devices complicated. DE 696 08 137 T2 discloses the provision of a read head that has a magnetization agent and at least one magnetoresistive element in which the magnetoresistive element has a variable resistor, which depends on the magnetic flux attached to it. The read head described therein is used such that each document of value to be detected during the relative movement of the document of value to the read head is first passed in front of the magnetization agent and then in front of said magnetoresistive element, wherein the magnetization agent can be designed such that a continuous magnetic flux is present at the sensor element.
Hard magnetic security features have a magnetic field themselves without needing an external magnetic field, provided the hard magnetic materials have been completely magnetized beforehand. This magnetic field also remains for a longer period of time. Over time, statistical processes occur that can demagnetize the hard magnetic materials. For example, when handled, bank notes are often impacted or creased. This can cause demagnetization of the hard magnetic materials. Therefore, to measure hard magnetic security features, it is helpful to imprint a new (obvious and permanent) magnetization feature onto the hard magnetic security feature using a pre-magnetization magnet. This newly applied magnetization can then retain the hard magnetic security feature over a longer timeframe, at least over the timeframe of the measurement.
A special security feature on bank notes is the ferromagnetic security thread (see DE 16 96 245 A1). In the industry, ferromagnetic materials that have a small coercive field strength and a large coercive field strength are used to produce the security thread. In a preferred embodiment, measuring devices to test such types of bank notes are therefore designed to detect both security threads made of material with low coercive field strength and security threads made of material with high coercive field strength.
WO 2010/006801 A1 discloses a measuring device for measuring the magnetic properties of the surroundings of the measuring device having a sensor line comprising at least two magnetoresistive sensor elements arranged in a line extending in a line direction and one support field device, which generates a magnetic support field, which has a magnetic field component pointing in a line direction and whose field strength in the line direction varies, wherein said field strength pattern has no zero-crossing and/or no maximum or minimum in the line direction on at least two sensor edges, arranged in succession in the line direction, of the sensor elements forming the sensor line. With the measuring device described therein, in a preferred embodiment, a pre-magnetizing magnet may be provided that is arranged in front of the sensor line. This pre-magnetizing magnet can be used to magnetize a measurement object with a hard magnetic pattern by means of which the measurement object is initially moved. If the measurement object is then moved via a sensor line, said sensor line can then detect the hard magnetic pattern.
In a preferred embodiment, the measuring device known from WO 2010/006801 A1 may have two sensor lines arranged one after the other in the read direction of the objects to be measured, and the sensor lines are used to determine the different features. One assumes in that case that a pre-magnetization of the bank note is necessary for detecting the magnetic pattern, and while the sensor line detecting the hard magnetic pattern cannot have any pre-magnetization field during the measurement, a strong pre-magnetization field is required for the sensor line at the site of the measurement to detect the soft magnetic pattern. With the embodiment described therein, hard magnetic and soft magnetic patterns can be detected in that the pre-magnetization field is effective in the area of a first sensor line and the magnetic features are practically completely magnetized thereby enabling detection of the soft and hard magnetic patterns and simultaneously serving to pre-magnetize the hard magnetic structures. If the bank note is subsequently routed above a second sensor line in an area in which the pre-magnetization field is no longer effective, this second sensor line can separately detect the purely hard magnetic pattern.